1. Field of the Invention
The present invention relates to an organic electroluminescence device (hereinafter abbreviated as organic EL device as needed) and a material for the organic electroluminescence device. In particular, the present invention relates to an organic electroluminescence device including a red emitting layer and a material used for the organic electroluminescence device.
2. Description of Related Art
An organic EL device, which includes an organic thin-film layer (in which an emitting layer is included) between an anode and a cathode, has been known to emit light using exciton energy generated by a recombination of holes and electrons that have been injected into the emitting layer (e.g., see document 1: US2002/0182441, document 2: WO2005/112519, document 3: JP-A-2003-142267, document 4: WO2007/046658, document 5: JP-A-2006-151966, document 6: JP-A-2005-8588, document 7: JP-A-2005-19219, document 8: JP-A-2005-197262, document 9: JP-A-2004-75567, document 10: US2008/0224603, document 11: JP-A-2004-281390, document 12: JP-A-2006-045503, document 13: WO2009/008215, document 14: WO2009/008205, document 15: WO2002/20693, document 16: JP-A-2001-250690, document 17: JP-A-2001-244075, document 18: JP-A-2001-257074, and document 19: JP-A-10-189248).
Such an organic EL device, which has the advantages as a self-emitting device, is expected to serve as an emitting device excellent in luminous efficiency, image quality, power consumption and thin design.
An example of a further improvement to be made in an organic EL device is an improvement in luminous efficiency.
In this respect, in order to enhance internal quantum efficiency, developments have been made on an emitting material (phosphorescent material) that emits light using triplet excitons. In recent years, there has been a report on a phosphorescent organic EL device.
Since the internal quantum efficiency can be enhanced up to 75% or more (up to approximately 100% in theory) by forming the emitting layer (phosphorescent-emitting layer) from such a phosphorescent material, an organic EL device having high efficiency and consuming less power can be obtained.
In forming the emitting layer, a doping method, according to which an emitting material (dopant) is doped to a host material, has been known as a usable method.
The emitting layer formed by the doping method can efficiently generate excitons from electric charges injected into the host material. With the exciton energy generated by the excitons being transferred to the dopant, the dopant can emit light with high efficiency.
In order to intermolecularly transfer the energy from the host material to the phosphorescent dopant, triplet energy EgH of the host material is required to be larger than triplet energy EgD of the phosphorescent dopant.
A known representative example of a material having effectively-large triplet energy has been CBP (4,4′-bis(N-carbazolyl)biphenyl). See, for instance, the document 1.
By using such CBP as the host material, energy can be transferred to a phosphorescent dopant for emitting light of a predetermined emitting wavelength (e.g., green, red), by which an organic EL device of high efficiency can be obtained.
Alternatively, the document 2 discloses a technique according to which a fused-ring derivative containing a nitrogen-containing ring such as carbazole is used as the host material for a red-phosphorescent-emitting layer.
On the other hand, a variety of host materials (fluorescent hosts) for fluorescent dopants that generate fluorescent emission are known. Various proposals have been made on a host material capable of, with a combination of a fluorescent dopant, providing a fluorescent-emitting layer excellent in luminous efficiency and lifetime.
However, although a fluorescent host has larger excited singlet energy Eg(S) than a fluorescent dopant, such a fluorescent host does not necessarily have larger triplet energy Eg(T). Accordingly, it is not successful to simply apply the fluorescent host to the host material (phosphorescent host) for a phosphorescent-emitting layer.
A well-known example of such a fluorescent host is an anthracene derivative.
However, triplet energy Eg(T) of an anthracene derivative is relatively small (approximately 1.9 eV). Thus, energy cannot be reliably transferred to a phosphorescent dopant for emitting light having a wavelength in a visible light range of 520 nm to 720 nm. In addition, excited triplet energy cannot be trapped within the emitting layer.
Accordingly, an anthracene derivative is not suitable for the phosphorescent host.
Further, derivatives such as a perylene derivative, a pyrene derivative and a naphthacene derivative are not preferable phosphorescent hosts for the same reason above.
Alternatively, an exemplary arrangement in which an aromatic hydrocarbon compound is used as the phosphorescent host has been known (the document 3). In the arrangement disclosed in the document 3, a compound in which two aromatic groups are bonded as substituents to a benzene central skeleton in meta positions is used as the phosphorescent host.
The documents 4 to 9 disclose organic EL devices in which various aromatic hydrocarbon compounds are used.
Further, the document 10 exemplifies compounds in which fused aromatic hydrocarbon rings are arranged at right and left substituent positions of 2,7-naphthalene rings.
The document 11 exemplifies compounds in which phenanthroline rings (nitrogen-containing heterocycles) are arranged at right and left substituent positions of 2,7-naphthalene rings.
The document 12 exemplifies compounds in which aromatic substituent groups of which essential skeletons are anthracene rings are arranged at right and left substituent positions of 2,7-naphthalene rings.
Moreover, the documents 13 and 14 disclose compounds having a structure in which four aromatic hydrocarbon rings are continuously coupled to one another and organic EL devices in which the compounds are used. Such an aromatic hydrocarbon compound is structured so that: the aromatic hydrocarbon ring at one terminal end is a fused polycyclic aromatic ring; the aromatic hydrocarbon ring adjoined and coupled to that aromatic hydrocarbon ring is a divalent benzene ring having a meta bonding; and the remaining two rings are fused aromatic rings. The documents further disclose that a device of which emitting layer uses both the aromatic hydrocarbon compound and a red-phosphorescent complex exhibits relatively high efficiency and relatively long device lifetime.
Further, fluoranthene compounds related to this invention and organic EL devices in which the fluoranthene compounds are used are also disclosed in the documents 15 to 19.
However, when applied with CBP as the host material, the organic EL device disclosed in the document 1 exhibits much higher luminous efficiency due to phosphorescent emission on one hand, but exhibits such a short lifetime as to be practically unusable on the other hand. Such a problem is considered to be attributed to considerable degradation of molecules by holes due to not-high oxidation stability that the molecular structure of CBP exhibits.
Although the organic EL device disclosed in the document 2 exhibits improvement in the luminous efficiency and lifetime, the improved luminous efficiency and lifetime may not be always sufficient for practical application.
The aromatic hydrocarbon compound disclosed in the document 3 is molecularly structured such that the molecules extend from the benzene central skeleton in a manner symmetrical relative to the benzene central skeleton. Therefore, an emitting layer applied with the aromatic hydrocarbon compound tends to be easily crystallized. Accordingly, an organic EL device in which the aromatic hydrocarbon compound disclosed in the document 3 is used may require higher driving voltage.
The documents 4 to 9 are totally silent on the effectivity of the aromatic hydrocarbon compounds used in organic EL devices as the phosphorescent hosts.
In the organic EL devices disclosed in the documents 10 to 14, which require relatively high driving voltage, lifetime of the devices may not be sufficiently prolonged.
Further, the documents 15 to 19 are totally silent on structures in which the fluoranthene compounds and phosphorescent materials are used together in organic EL devices.